Recovering traffic with minimal loss is a fundamental requirement in carrier-class networks. Fast-Reroute (FRR) is a technique to recover traffic with minimal loss under failure conditions in a network.
LDP (Label Distribution Protocol), defined in RFC 5036, is a widely deployed protocol to setup Label Switched Paths (LSPs) in MPLS (MultiProtocol Label Switching) (defined in RFCs 3031 and 3032) implementations. LDP establishes LSPs along routed paths setup by IGP (Interior Gateway Protocol) (defined, for example, in RFC 2328). Thus, the convergence of LSPs established with LDP under failure conditions is gated by IGP convergence.
RSVP-TE (Resource Reservation Protocol—Traffic Engineering) based FRR has been standardized (RFC 4090) and implemented in several vendors platforms. Some operators and vendors have tried to address the fast-convergence of LDP by using RSVP-TE. This feature is typically referred to as LDP-over-RSVP.
Since LDP follows routed paths setup by IGP, its convergence is gated by IGP convergence. However IGP convergence has been traditionally slow. A good description of the problem is in section 4 of RFC 5714. For example, such reasons include: the time taken to detect the failure, the amount of time for the local router to react the failure, the amount of time to transmit the information about the failure to other routers in the network, the amount of time to re-compute the forwarding tables, and the amount of time to download the re-computed forwarding tables into the forwarding hardware. Several approaches have tried to introduce FRR in IGP to improve IGP convergence, but each of them have been plagued by several problems. For example, approaches to solving this problem such as draft-ietf-rtgwg-ipfrr-notvia-addresses-OX has deployment and implementation complexity and hence has not been adopted. Approaches such as Loop Free Alternates (described in RFC 5286) do not have full coverage, hence carriers have reservations in deploying them.
Another approach to providing FRR for LDP LSPs is to use RSVP-TE as a failure-bypass mechanism (LDP-over-RSVP). However, carriers have been slow to deploy RSVP-TE due to several reasons, including the extensive configuration and maintenance experience requirements since an additional, fairly complex protocol such as RSVP-TE is used, leading to increased operating expenses. LDP-over-RSVP also requires the vendor to support many features (such as high availability and reliability) in RSVP-TE that may not be available in many implementations.